Theoretical Investigation of the Role of the Nitride Ion in the Magnetism of Oxynitride MnTaO2N

• Published in: Journal of physical chemistry. C 2019-10, Vol.123 (41), p.25379-25384
• Main Authors: Mo, Shishin, Kurauchi, Yuji, Katayama, Tsukasa, Hirose, Yasushi, Hasegawa, Tetsuya
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b07446

Oxynitride MnTaO2N exhibits a helical spin order in contrast to the isostructural oxide MnTiO3 with a G-type antiferromagnetism. To understand the role of the nitride ions on the magnetism, in this study, we theoretically investigated the structural and magnetic properties of MnTaO2N. Band calculations based on the density functional theory revealed that besides the most stable anion coordinations of cis-MO4N2 octahedra (M = Mn and Ta), the other coordinations such as trans-MO4N2, MO3N3, and MO5N are also considered to coexist due to the small total energy difference. This results in random existence of the nitride ions in MnTaO2N. The magnetic properties were investigated using the Heisenberg model to quantify the coupling energy (J) between Mn 3d5 local moments. Interestingly, the average J of the Mn–N–Mn bonds (J 1N) was four times larger than that of the Mn–O–Mn bonds (J 1O), mainly due to the covalent nature of Mn–N bonds. In comparison to MnTiO3, which contains only one type of magnetic interaction, J 1O, MnTaO2N has various kinds of magnetic interactions due to the random existence of the nitride ions. In addition, the J 1O value became comparable to the next-nearest-neighbor interaction (J 2). These results caused spin frustration, which is a prerequisite to the emergence of the helical spin order in MnTaO2N.